In situ hybridization offers a precise method for the definition of specific chromosomal regions. Localization of specific gene sites will be carried out primarily in the human, although other primates will be investigated in terms of evolutionary comparisons. As a corollary study, the possibility of specific viral insertion sites on the human chromosome will be investigated. The effectiveness of hybridization in situ depends on the availibility of purified probe molecules of sufficient specific activity to localize the homologous chromosomal region. Several methods for obtaining highly radioactive RNA or DNA have been developed for in situ hybridization studies. Appropriate experimental procedures for isolation and purification of specific RNA or DNA fractions are discussed.